In the main trend of society such as global environmental preservation, the importance of electronic businesses for reducing an environmental load comes to the fore. Particularly, a power device is used in an inverter device of a railroad vehicle or a hybrid/electric vehicle, an inverter device of an air conditioner, and a power source of consumer equipment such as a personal computer. The improvement in performance of the power device contributes to the improvement in efficiency of the power conversion of an infrastructure system and the consumer equipment. The improvement in the power conversion efficiency means a reduction of energy resources necessary for the operation of the system. In other words, the amount of carbon dioxide emission can be reduced; that is, an environmental load can be reduced. Therefore, research and development for improving the performance of the power device are actively performed by companies.
In general, the power device is made of a silicon material (Si) similar to a large-scale integrated circuit (LSI). In a power conversion device (the inverter device and the like) using such a Si power device, the development for realizing a low ON resistance (Ron), a high current density, and a high breakdown voltage is actively performed such that an element structure of diodes and switch elements and a profile of impurity concentration are optimized in order to reduce the energy loss caused in the inverter device.
In recent years, compound semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) having a bandgap larger than silicon are drawing an attention as materials of the power device. Since the compound semiconductor has a large bandgap, a breakdown voltage thereof is 10 times larger than that of silicon. Therefore, a compound device can be made thinner than a Si device, and a resistance (Ron) during conduction is significantly lowered. As a result, a so-called conduction loss (Ron·i2) obtained by multiplying the resistance (Ron) by the conduction current (i) can be reduced, and thus it can attribute to improvement in power efficiency. Paying attention to such an advantage, the diode and the switch element using the compound material are actively developed in domestic and foreign countries.
As an application of the power device, for example, a converter device of a synchronous rectifying type and an inverter device of a DC/AC conversion device are generally exemplified. Describing the inverter device in simple, two sets of the switch elements of the power device with a freewheel diode are connected in series between a power source on the high voltage side (upper arm) and a power source on the low voltage side (lower arm). A DC level at the previous stage of the inverter device is converted into an AC level by alternately turning on and off the switch elements of the upper and lower arms so as to be supplied to a load circuit such as an AC insulating transformer or a motor at the following stage. Further, a dead time generation circuit is included in a gate drive circuit to control the switches of the upper and lower arm to be alternately turned on and off (that is, the switch elements of the upper and lower arms do not enter the ON state at the same time). Similarly in the converter device of the synchronous rectifying type, the switch elements of the upper and lower arms are not simultaneously turned on since the dead time is set to be sufficiently long in general. However, when the dead time is unnecessarily lengthened, a conductive loss component of the diode is increased, and the power conversion efficiency of the inverter device or the converter device may be worse. On the other hand, when the dead time is excessively shortened, the switch elements of the upper and lower arms may enter the ON state at the same time, and thus a significantly large through current flows from the high potential power source to the low potential power source. Therefore, the switch elements of the upper and lower arms may be broken. For this reason, an optimal minimization of the dead time of the inverter device and the converter device is an important point for reducing the loss in the power conversion device. Further, in a case where the ON resistance of a main switch element is small to be several mΩ, and a reverse conduction voltage of the main switch element is lower than the ON voltage of the diode at the time of freewheeling (an application of the power source voltage of about tens of V), the effect of loss reduction by the minimization of the dead time is increased.
For example, Patent Document 1 discloses a configuration in which level shift circuits are included in both of a high-side driver and a low-side driver in order to optimize the dead time in a DC/DC converter of the synchronous rectifying type.